The present invention generally relates to failure detection systems, and more particularly to a failure detection system for detecting a failure of a package in a redundant system having a duplex package configuration.
In systems such as a communication processing unit which must operate continuously without interruption, the continuous operation of the system is guaranteed by employing a duplex package configuration and automatically selecting the package in the active (or operating) system. In such a system, it is desirable to enable replacement of the package without stopping the operation of the system and without affecting the operation of the system.
Therefore, there is a demand to realize a failure detection system which enables replacement of the inactive (or standby) package during system operation, that is, in the state where the power source is turned ON, without affecting the operation of the system.
FIG. 1 shows an example of a conventional failure detection system, and FIG. 2 shows the circuit construction of a switch processing part shown in FIG. 1.
In FIG. 1, an output signal of a circuit (not shown) which is carried on a system-0 package B0 is branched and applied to a first input terminal of each of selectors 1 and 7 which are carried on respective packages C0 and C1. The packages C0 and C1 are provided in a stage subsequent to the stage in which the packages B0 and B1 are provided. Similarly, an output signal of a circuit (not shown) which is carried on the system-1 package B1 is branched and applied to a second input terminal of each of the selectors 1 and 7 which are carried on the respective packages C0 and C1.
In the package C0, the output signals of the packages B0 and B1 which are input to the selector 1 are respectively branched and applied to alarm detectors 3 and 4. The alarm detectors 3 and 4 monitor the respective input signals and output alarms A1 and A2 to a switch processing part 9 if a failure is detected.
Similarly, in the package C1, the output signals of the packages B0 and B1 which are input to the selector 7 are respectively branched and applied to alarm detectors 5 and 6. The alarm detectors 5 and 6 monitor the respective input signals and output alarms A3 and A4 to the switch processing part 9 if a failure is detected.
The switch processing part 9 outputs selection signals for switching the selectors 1 and 7 based on the alarms A1 and A2 from the package C0 and the alarms A3 and A4 from the package C1, and depending on the state of the alarms as shown in Table 1. The selectors 1 and 7 respectively output the input signals from one of the packages B0 and B1 in response to the respective selection signals.
TABLE 1 ______________________________________ State of Alarm A1 A2 A3 A4 State of Selection ______________________________________ (1) O O O O previous System O O O X System-0 (3) O O X O System-1 O O X X -- O X O O System-0 O X O X System-0 O X X O -- O X X X -- X O O O System-1 X O O X -- X O X O System-1 X O X X -- X X O O -- X X O X -- X X X O -- (2) X X X X System-0 ______________________________________
In Table 1 above, "O" indicates the normal state where no alarm is generated, and "X" indicates the state where the alarm is generated. A description will not be given of the cases where the alarm is or is not generated, by referring to FIG. 2 which shows the circuit construction of the switch processing part 9.
In FIG. 2, in the normal state where none of the alarms A1 through A4 are generated as indicated by (1) in Table 1, for example, a signal "0" indicating the normal state is converted into a signal "1" via inverters 20, 21 and 23 and is applied to an AND circuit 24. Since all of the four input signals of the AND circuit 24 are "1" a signal "1" is output from the AND circuit 24. The signal "1" from the AND circuit 24 is applied to a terminal a of a selector 33 via an OR circuit 31. On the other hand, because output signals of AND circuits 27 and 30 are both "0" a signal "0" is applied to a terminal b of the selector 33 via an OR circuit 32.
Table 2 shows the relationship of the input signals and the output signal of the selector 33. In the above described case, the signals "1" and "0" are respectively applied to the terminals a and b of the selector 33. Hence, as may be seen from Table 2, the selector 33 selects and outputs the signal from an input terminal T2 and applies this signal to a terminal D of a flip-flop circuit 34. The flip-flop circuit 34 outputs this incoming signal from the terminal D from an output terminal Q in response to a clock which is applied to a terminal C. The output signal of the flip-flop circuit 34 from the terminal Q is applied to the selectors 1 and 7 shown in FIG. 1 as the selection signals. In this case, the switch processing part 9 feeds back the signal from the terminal Q of the flip-flop circuit 34 to the input terminal T2 of the selector 33, so that the signal is output again via the selector 33 and the flip-flop circuit 34. Hence, as indicated by (1) in Table 1, the state of the immediately previous system is selected.
TABLE 2 ______________________________________ Terminal a Terminal b Selection State ______________________________________ "0" "0" Select "0"-input "0" "1" Select T1-input "1" "0" Select T2-input "1" "1" -- ______________________________________
On the other hand, in the case where all of the alarms A1 through A4 are generated as indicated by (2) in Table 1, a signal "1" which indicates the generation of the alarm is inverted into a signal "0" before being applied to each of AND circuits 11, 13, 15, 17, 19, 24, 27 and 30 shown in FIG. 2. Hence, a signal "0" is applied to both the terminals a and b of the selector 33. From Table 2, when the signal "0" is applied to both the terminals a and b of the selector 33, the selector 33 selects the "0"-input and applies this signal "0" to the terminal D of the flip-flop circuit 34.
The signal "0" applied to the terminal D is output from the terminal Q of the flip-flop circuit 34 responsive to the clock which is applied to the terminal C. The output signal of the flip-flop circuit 34 from the terminal Q is applied to the selectors 1 and 7 shown in FIG. 1 as the selection signals. As a result, the selectors 1 and 7 select the system-0, that is, select and output the input signal from the package B0 to a processor 2 of the package CO.
In cases where the alarms A1 through A4 assume states other than those described above, the immediately previous system, the system-0 or the system-1, is selected depending on the states of the alarms A1 through A4 shown in Table 1.
For example, when the output signal of the package B0 is branched and input to the alarm detectors 3 and 5 of the packages C0 and C1, a failure may be detected in one of the alarm detectors 3 and 5 but not in the other. Such a case is indicated by a symbol "--" in Table 1 under the column "State of Selection". Such a case normally does not occur, but the immediately previous system may be selected if it does occur. Here, the "immediately previous system" refers to the package which is of the same system-0 or system-1 to which a certain package belongs but is located at a stage immediately preceding the certain package.
Conventionally, the switching of the packages was made in response to the failure detection in the above described manner.
However, according to the conventional failure detection system, the switch processing part 9 carries out a switching process when a failure is detected in the active package, regardless of which package is active and which package is inactive. In other words, if it is assumed for the sake of convenience that the packages B0 and C0 of the system-0 are active in FIG. 1, a failure of the package B0 may be erroneously detected in the package C1 when the package C1 is disconnected or connected for replacement purposes. However, the switch processing part 9 in this case has no means of determining whether or not the alarm A3 from the package C1 is generated in error. As a result, the switch processing part 9 controls the selectors 1 and 7 of the packages C0 and C1 so as to select the system-1 in response to the alarm A3.
Therefore, although the package B0 of the system-0 is operating normally in this case, a switching is made by the switch processing part 9. Consequently, there is a problem in that the active package and the operation of the active package are affected when the inactive package is disconnected or connected for replacement purposes.